1. Field of the Invention
The present invention relates to an innovative electromagnetic actuator and a lens drive mechanism using the same. Specifically, the present invention is intended to provide an electromagnetic actuator which slides a movable member including a driven body of circular radial cross section in the axial direction inside a cylindrical casing, and provide a lens drive mechanism using the actuator. The inventive actuator and mechanism are suitably used for a drive mechanism for moving a focusing lens or zooming lens inside the lens barrel equipped on a video camera or the like. The inventive actuator and mechanism are designed to be compact by eliminating unneeded spaces, while exerting a large drive force based on the enhanced magnetic efficiency.
2. Description of Related Art
Generally, a video camera or the like having the auto-focusing function or electric zooming function has its lens barrel equipped with drive means for moving a movable lens of auto-focusing or zooming in the direction of its optical axis. Drive means of this kind often employ electromagnetic actuators including a set of a coil and a magnet.
FIGS. 6, 7 and 8 show an example of such electromagnetic actuators, which is used to move a focusing lens. The electromagnetic actuator indicated by "a" consists of a fixed member c disposed in a casing b having a shape of a rectangular cylinder, and a movable member d which is supported by being in non-contact with the fixed member c and slidable in the axial direction inside the casing b. The casing b is provided within it with two supporting rails e extending along two confronting corners and in parallel to the central axis of the casing b.
The fixed member c consists of a yoke member f made of magnetic material and magnets g fixed on the yoke member f. The yoke member f is an integrated member made up of an inner yoke h having a shape of rectangular cylinder which is smaller than the yoke member f, a connecting flange i which roots on the rear edge of the inner yoke h and extends outward, and four outer yokes j which root on the the outer edge of the connecting flange i and extend along the inner yoke h. Each magnet g is fixed on the inner side of the outer yoke j to face the inner yoke h. The magnets g produce a magnetic field k in the space between the magnets g and the inner yoke h.
The movable member d consists of a metallic frame 1 having a generally rectangular front face, with a relatively large circular opening being formed at the center, a lens holder m having a shape of circular cylinder which roots on the edge of the opening of the frame 1 and extends forward, a focusing lens n held by the lens holder m, and a coil bobbin o having a shape of rectangular cylinder which roots on the outer edge of the frame 1 and extends rearwardly.
The coil bobbin o has its radial cross-sectional dimensions made slightly larger than those of the inner yoke h and slightly smaller than the rectangle that is defined by the inner surfaces of the four magnets g. The coil bobbin o consists of a rear bobbin section p, with a groove being formed on its outer surface and a coil r being wound in the groove coaxially with the optical axis, and a connecting section q which connects the bobbin section p to the frame 1.
Indicated by s is one of two support arms extending radially in diagonal directions from the outer surface of the lens holder m and reach radial positions beyond that of the corners of the frame 1. Bearing sections t are formed in the arms at the positions corresponding to the supporting rails e. The support arms s engage with the supporting rails e by means of the bearing sections t so that the movable member d is supported slidably in the direction of the optical axis inside the casing b, with the coil bobbin o and coil r being exposed to the magnetic field k produced between the inner yoke h and magnets g. When a drive current is supplied to the coil r, a drive force is generated in a direction depending on the current direction, and the movable member d moves in the direction of optical axis.
However, the foregoing conventional electromagnetic actuator necessitates the connecting section q of the coil bobbin o for providing a clearance so that the frame 1 of the movable member d does not hit the front end of the yoke member f when the coil r is brought to the rearmost position in its moving range, thereby resulting unfavorably in an increased axial dimension of the movable member d. Consequently, the focusing lens n appears to come out forward too much when the movable member d is located at the foremost position in its moving range, resulting unfavorably in an increased axial dimension of the overall electromagnetic actuator.
Moreover, the fixed member c and movable member d having a rectangular radial cross section take an extra volume for their corner sections, which do not match with the focusing lens n having a circular radial cross section, and hinder the compact and light-weight design. Due to the fixed member of this shape, four divided magnets must be used, thus hindering the improvement of the magnetic efficiency.
Moreover, the planar outer yokes j extending from the outer edge of the connecting flange i are liable to bend at the root, and in this event the movable member d may come to contact with a bent outer yoke j. Although a conceivable precautionary design is to allow much clearance between the inner yoke h and outer yokes j, an excessive clearance will deteriorate the magnetic efficiency and thus reduce the drive force. This problem is more pronounced as the moving range of the movable lens is increased, and therefore it is difficult for this structure to be applied to lens systems having a long moving range, such as a zoom lens system, or it is difficult to accomplish a zoom lens system having a high magnification factor.
Moreover, the coil bobbin o having a rectangular radial cross section is liable to cause the coil r to stray from the bobbin surface at its linear sections, and in this event the coil r may come to contact with the inner yoke h.
The foregoing prior art problems are more pronounced as the moving range of the movable member d is increased, and it has been desired to dissolve these problems.